Ship-towed hydrophone volumetric array system apparatus

ABSTRACT

This invention provides a system apparatus and method for ship-towed deployment of a non-linear volumetric array of hydrophones, allowing line-intersect or line-transect sampling of marine mammal populations through passive acoustic monitoring, enabling unambiguous real-time three-dimensional localization of single sounds received through a low-cost, modular, robust, stable, small, light, neutrally to slightly negatively buoyant volumetric array having low self-noise and low flow noise, that avoids putting high tension on the tow cable and that is compatible with standard hydrophones, instrumentation, cabling, and analytical software.

This invention was made with Government support under ContractWC-133R-15-CN-0079 awarded by NOAA. The Government has certain rights inthe invention.

BACKGROUND

This invention provides a system apparatus and method for a low-dragship-towed deployment of a non-linear volumetric array of hydrophones,allowing line-intersect or line-transect sampling of marine mammalpopulations, where “ship-towed” is defined as any type of vehiclecapable of moving the apparatus through the water.

Passive acoustic monitoring (PAM) is the preferred technique fordetecting marine mammals, because the marine mammals use low-frequencysounds for their own echo-location and communication, and the passivemonitoring does not interrupt nor distort those sounds. Thelow-frequency sounds are very efficiently transmitted in seawater andtravel great distances because the hydroacoustic impedance properties ofseawater favor lower frequencies and disfavor higher frequencies.

Fixed-location hydrophones have been used to monitor marine mammals, butcannot monitor the vast amounts of deep-ocean habitat as required.Presently, line arrays of hydrophones, towed one behind the other, areused. In order to obtain useful location information, the line arrayneeds to detect more than a single isolated click in order to obtainenough information to make a triangulation. A massive amount of datafrom line arrays must be processed and analyzed after the fact in orderto derive the location information.

The community organization PAMGUARD has been established to address thefundamental limitations of existing cetacean passive acoustic monitoring(PAM) software capabilities, and continues to develop open-source PAMsoftware for acquiring and analyzing hydroacoustic data related tomarine mammals. Any equipment or methods developed for this field shouldbe operable or interoperable with this evolving PAMGUARD software.

Sound will reflect at the interface boundary of materials with differingacoustic impedance. Sound will also attenuate in materials. Hydrophones,when towed in the ocean, need a boundary layer to separate thehydrophone from the water flow or significant flow noise will result. Inorder to optimize hydrophone measurements in the ocean environment, oneshould surround the hydrophones with materials having low attenuationand close acoustic impedance matches to seawater.

The towing of a hydrophone array across great distances at a reasonablespeed to cover those distances, which is about 10 knots, puts a hugeamount of stress on any hydrophone array, whether a line array orotherwise, and on any tow cable used. In order to tow the hydrophonearray far enough behind the towing ship to avoid the hydroacoustic noiseof the ship, the tow cable needs to be at least 100 meters long, andpreferably 300 meters long. Such tow cables, which are integrated withdata-transfer cables, are known, and any new equipment or methods shouldmake use of such existing tow cables, if possible. The existing cablescan withstand 1000 pounds of tension, and that breaking point could bereached if the towed hydrophone array generated even a few hundredpounds of hydrodynamic drag force.

The monitored sounds are very faint, and can be overwhelmed byturbulence in the vicinity of the hydrophones. Existing practice of thePAM technique suffers from turbulence caused by cable drag and byhydrodynamic drag or turbulent flow around whatever housing is providedfor a hydrophone array being towed at 10 knots at the end of a 300 metertow cable.

Because the PAM technique requires detection of the small differences inthe time of a given sound reaching each hydrophone in an array, thelatitude, longitude, and depth position of the whole array, plus theposition of the individual hydrophone with respect to one another, mustbe known in order to analyze the data, and preferably should be stablein order to avoid additional complexity in the analysis, and becausesuch instability is likely correlated with hydrodynamic drag,turbulence, self-noise, and strain on the hydrophone array structure andthe tow cable.

The advantages of an array of hydrophones arranged in a tetrahedron areknown. Each hydrophone is equidistant from each other hydrophone,simplifying the calculations, and the relative position of eachhydrophone to the others is fixed. However, a tetrahedron is difficultto tow underwater at the end of a long cable, and any roll, pitch, oryaw in the travel of the hydrophone array will alter the relativeposition of each hydrophone with respect to the underwater sound source.Any such change of orientation or attitude of the hydrophone array mustbe captured and accounted for in the analysis of the data. Such changesof orientation or attitude are also likely to correlate with increasedhydrodynamic drag, self-noise, and strain on the hydrophone arraystructure and the tow cable.

There is a need for a small, low cost volumetric array, integrated withPAMGUARD, for the use of the government, military, and universities formarine mammal population studies, mitigation for military and commercialactivities in the ocean, and detection and localization of submergedassets such as downed planes, moorings, AUVs or ROVs.

SUMMARY OF THE INVENTION

This invention provides a system apparatus and method for ship-toweddeployment of a non-linear volumetric array of hydrophones, allowingline-intersect or line-transect sampling of marine mammal populationsthrough passive acoustic monitoring, enabling unambiguous real-timethree-dimensional localization of single sounds received through alow-cost, modular, robust, stable, small, light, neutrally to slightlynegatively buoyant volumetric array having low self-noise and low flownoise, that avoids putting high tension on the tow cable and that iscompatible with standard hydrophones, instrumentation, cabling, andanalytical software.

BRIEF DESCRIPTION OF DRAWINGS

Reference will now be made to the drawings, wherein like parts aredesignated by like numerals, and wherein:

FIG. 1 is a schematic view of the ship-towed hydrophone volumetric arraysystem of the invention in use;

FIG. 2 is an orthographic axonometric view of the ship-towed hydrophonevolumetric array system of the invention;

FIG. 3 is a section view of the fin profile of the ship-towed hydrophonevolumetric array system of the invention;

FIG. 4 is an orthographic axonometric view of the ship-towed hydrophonevolumetric array system of the invention;

FIG. 5 is a front view of the ship-towed hydrophone volumetric arraysystem of the invention;

FIG. 6 is a back view of the ship-towed hydrophone volumetric arraysystem of the invention;

FIG. 7 is a bottom view of the ship-towed hydrophone volumetric arraysystem of the invention;

FIG. 8 is a top view of the ship-towed hydrophone volumetric arraysystem of the invention;

FIG. 9 is a side view of the ship-towed hydrophone volumetric arraysystem of the invention;

FIG. 10 is a section view of the ship-towed hydrophone volumetric arraysystem of the invention;

FIG. 11 is a schematic view of the internal components of the ship-towedhydrophone volumetric array system of the invention;

FIG. 12 is an exploded view of the ship-towed hydrophone volumetricarray system of the invention;

FIG. 13 is a schematic view of the ship-towed hydrophone volumetricarray system of the invention in trim;

FIG. 14 is a schematic view of the ship-towed hydrophone volumetricarray system of the invention rolling;

FIG. 15 is a schematic view of the ship-towed hydrophone volumetricarray system of the invention yawing;

FIG. 16 is a schematic view of the ship-towed hydrophone volumetricarray system of the invention pitching; and

FIG. 17 is a schematic view of the shipboard or topside operation of theship-towed hydrophone volumetric array system of the invention in use.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the ship-towed hydrophone volumetric array systeminvention method 100 and apparatus 10 are illustrated schematically. Theactivity is line-intersect or line-transect sampling of marine mammalpopulations through passive acoustic monitoring. The water is deep andthe area covered is wide, so the underwater sound of the ship and of anyequipment used must be minimized in order to be able to pick up thedesired signals. The hydrophone volumetric array system is towed behinda ship on a cable about 300 meters in length, in order to getsufficiently far away from the ship's noise. The tow speed is about 10knots, because so much area must be covered in a single run in order tocollect sufficient data.

An underwater sound wave from a single click from a whale propagatesthrough the water and hits each individual hydrophone 1, 2, 3, 4 in theship-towed hydrophone volumetric array system 10 apparatus at slightlydifferent times correlated with the slight differences in straight-linedistances to the sound source. Aboard the towing ship are the topsideelectronics 90, receiving signals from a GPS antenna 91 andcommunicating with the hydrophone volumetric array through amulti-channel power and data cable 92 incorporated into or onto the towcable 93. The topside electronics are able to analyze and display inreal time the signals being received from the hydrophone array and thelocation of the sound source in relation to the hydrophone array, thelocation of which is known from applying the proper corrections to theGPS data. The result is real-time, three-dimensional locationinformation.

Referring to FIG. 2, the four hydrophones 1, 2, 3, 4 are fixed in atetrahedral relationship one to the other in the ship-towed hydrophonevolumetric array 10 system apparatus. With reference to the direction oftravel, there is a fore hydrophone 1, aft hydrophone 4, port hydrophone2, and starboard hydrophone 3. The four hydrophones generate data thatis consolidated and amplified in an electronics housing 5, which alsoconsolidates and amplifies data from a pressure-depth sensor 6 and anorientation sensor 7, and all of the data is sent through the taperedcable clamp 65 through the tow cable and to the topside electronics.

The ship-towed hydrophone volumetric array 10 system apparatus isstreamlined to eliminate hydrodynamic drag and to enhance stability asmuch as possible. Structural components are the fore-structure 20,aft-structure 30, and mid-structure 40. The mid-structure 40 has a maintube housing 41 enclosing the electronics housing 5, pressure-depthsensor 6 and orientation sensor 7. The central axis of the main tubehousing 41 passes through the projected center point of the tetrahedronof hydrophones. This central axis is also the axis of the direction oftravel of the whole unit. There are three hydrophone tubes 42, 43, 44arrayed around the main tube housing 41 with 120 degrees separation ofone tube to the others, and located at 60 degrees, 180 degrees, and 300degrees around the central axis of the main tube housing 41. The threehydrophone tubes are the fore-and-aft hydrophone tube 42 located at 180degrees or directly below the main tube housing in use. The porthydrophone tube 43 and the starboard hydrophone tube 44 are located at60 degrees and 300 degrees.

The central axis of the main tube housing 41 passes through theprojected center point of the tetrahedron of hydrophones, with theconsequence that the fore and aft hydrophones 1, 4 and the center pointare in a plane along the central axis, and the port and starboardhydrophones 2, 3 and the center point are in a plane perpendicular tothe central axis. The distance between the central axis of the main tubehousing 41 and the central axis of the fore-and-aft hydrophone tube 42is therefore one-fifth shorter, or 80% of the corresponding distance forthe port and starboard hydrophone tubes 43, 44.

The fore-structure 20 functions as fairing and stabilization of themid-structure 40 and is attached in smooth continuations of the cylinderand tubes of the mid-structure 40, both as to continuation of therelevant central axes and to continuation of outer surfaces, in order toavoid turbulence and hydrodynamic drag. The fore-structure 20 has astreamlined main nose cone 21 on the main body and three subsidiary nosecones 22, 23, 24 affixed to fore-fins 25, 26, 27 that connect thestreamlined main nose cone 21 to the three subsidiary nose cones 22, 23,24. The nose cones 21, 22, 23, 24 and fore-fins 25, 26, 27 are hollow orpartially hollow to provide paths for wiring from the hydrophones to theelectronics housing 5, and for lighter weight. The three subsidiary nosecones are the same size and are designated as fore-and-aft nose cone 22,port nose cone 23, and starboard nose cone 24. The three fore-fins arenot all of equal length from the main nose cone to the subsidiary nosecone. The fore-and-aft fore-fin 25 is shorter than the port andstarboard fore-fins 26, 27, which are of length equal one to the other.

The aft-structure 30 supports and stabilizes the hydrophone tubes 42,43, 44, and, with the fore-structure 20, stabilizes the ship-towedhydrophone volumetric array system 10 apparatus in use. As with theattachment of the fore-structure 20 and mid-structure 40, the attachmentof the mid-structure 40 and aft-structure 30 smoothly continues therelevant central axes and outer surfaces. The aft-structure 30 has amain open aft collar 31 and three subsidiary open aft collars, afore-and-aft open aft collar 32, a port open aft collar 33, and astarboard open aft collar 34, with each subsidiary open aft collarconnected to the main open aft collar 31 by the fore-and-aft aft fin 35,port aft fin 36, and starboard aft fin 37, corresponding to the relevantportions of the fore-structure 20 and mid-structure 40.

The main open aft collar 31 and the three subsidiary open aft collars32, 33, 34 are all open cylinders or tubes, which allow water to fillall of the mid-structure 40 and the hollow portions of thefore-structure when the invention is in use under water, and to drainout of those structures when the invention is retrieved or handledtopside out of the water. It is highly desirable to fill the ship-towedhydrophone volumetric array system 10 with water when in use, becausethe water continues the transmission of sound to the hydrophones, whereair would not and where any hydroacoustically different substance wouldcause reflection of the sound waves at the boundaries. Also, the smooth,stable operation of the hydrophone array under water requires that thehydrophone array, when in use, has neutral buoyancy or slightly negativebuoyancy of five pounds negative plus or minus five pounds. Otherwise,any tendency to float, combined with the natural tendency of the towcable itself to rise when pulled at significant speeds, is likely toprevent the hydrophone array from staying consistently at the desireddepth, which is on the order of ten meters deep. With neutral orslightly negative buoyancy of the hydrophone array, depressor weightsonly in the amount needed to counteract the effects of cable drag, or todampen cable strum, will be required, with none to very little depressorweight needed for the hydrophone array. The addition of solid weight tothe structure of the hydrophone array, or filling portions with aheavier-than-seawater liquid, would increase the topside weight andwould detrimentally dampen the hydroacoustic transparency of thehydrophone array.

The topside or dry weight of a preferred embodiment of the ship-towedhydrophone volumetric array 10, built with the preferred materials andequipped with the preferred equipment, is approximately 30 pounds, andthe size is 110 centimeters by 55 centimeters, just over one meter byone-half meter. The size and weight make the ship-towed hydrophonevolumetric array 10 easy for one person to handle, deploy, and retrieve.

With the water entering through the open aft collars, abaft thehydrophones, and with no significant water entering through thefore-structure, there is no water actively flowing past the hydrophonesinside the hydrophone tubes, but instead there is an envelope ofrelatively still water in direct contact with the hydrophones. Thisprovides an advantage because the seawater inside the hydrophone tubeshas the same hydroacoustic properties as the seawater outside the tubes,but the noisy effects of having the seawater constantly flowing directlyover the hydrophones is avoided. The open collars on the aft end do notpresent any significant hydrodynamic perturbances.

A tapered cable clamp 65 is provided at the center of the tip of themain nose cone 21 for the purposes of towing, low-voltage power, anddata communications through data cables incorporated into or onto thetow cable.

Referring to FIG. 3, illustrating the streamlined NACA 0012 wing or finprofile, the cross sectional profile of the fore-fins 25, 26, 27 andaft-fins 35, 36, 37 must be streamlined to promote laminar flow andsuppress turbulent flow, in order to avoid the generation of noise at orahead of the hydrophones, which the hydrophones will subsequently bepulled through. Suppression of turbulence behind the hydrophones issomewhat less critical, but still necessary since any resulting noisetravels at much greater speed than 10 knots, and will reach thehydrophones. In a preferred embodiment of the invention, the streamlinedfore-fins and aft-fins have the NACA 0012 profile.

Referring to FIG. 4, the ship-towed hydrophone volumetric array 10system optionally provides a cable-connector fairing boot 66 for thepurpose of streamlining or fairing the tow-cable connection andintegrating its fairing effect with the fairing effect of the main nosecone 21. The main nose cone 21 has a spherically blunted tangent ogiveprofile, which minimizes hydrodynamic drag in that area upon which agreater amount of pressure is concentrated. Fairing is provided at thejoining of nose cones and fins in the fore-structure 20 and of collarsand fins in the aft-structure 30.

Referring additionally to FIG. 5 & FIG. 6, the four hydrophones aresupported in a tetrahedral relationship, by definition with equalstraight-line distances between any given two hydrophones. In apreferred embodiment of the invention, that distance is 50 centimeters,the equivalent of one-half meter. That distance will figure into thevolumetric or three-dimensional analysis of data from the hydrophonearray. It has been found to be a sufficient distance for obtainingmeaningful and quantifiable differences in the arrival of sound waves.Based on such a 50-centimeter separation, the overall longitudinallength of the central or main portion of the hydrophone array embodimentis 110 centimeters, the diameter of the main tube housing 41 is 6centimeters, and the diameter of the three subsidiary hydrophone tubes42, 43, 44 is 5 centimeters each. Measuring from the central axes ofeach tube, which are parallel each to the others, the distance from thecenter of the main tube housing 41 to the center of the port tube 43 is25 centimeters, with the distance to the starboard tube 24 being thesame. The distance to the center of the fore-and-aft tube 42 is 20centimeters, which is 80% of the other distances. A functioningfull-scale prototype of the preferred embodiment was built usingschedule-80 pipe of 4-inch and 1.5-inch ID size for the main central andthe subsidiary tubes.

Referring to FIG. 7 & FIG. 8, the bilateral symmetry along the centralaxis in the bottom and top views, and the two-dimensional spatialrelationships among the hydrophones, can be seen. Referring to FIG. 9,the asymmetry along the central axis caused by the accommodation of thetetrahedral array in the three-dimensional structure of the ship-towedhydrophone volumetric array system 10 apparatus, and the two-dimensionalspacial relationships among the hydrophones can be seen.

Referring to FIG. 10 & FIG. 11, the tapered cable clamp 65 has a tapercorresponding to a taper built into the main nose cone 21. The taperedcable clamp 65 grips the tow cable 93 and incorporated power and datacable 92. The force of the towing tightens the connection because of thetaper. The tow cable 93 and incorporated power and data cable 92 is thenconnected to the electronics housing 5 leaving a small amount of slackin the cable, providing strain relief and a backup cable attachment thatcan be used for a slow retrieval of the ship-towed hydrophone volumetricarray in the event of a failure of the tapered cable clamp. Optionally,a cable-connector fairing boot 66 can be provided to further streamlinethe assembly.

Appropriate hydrophones are the BENTHOWAVE BII-7094, with a sensitivityof −204 dBV/uPas a capacitance of 13 nF, and the HTI HTI-96-MIN, with asensitivity of −201 dBV/uPas and a capacitance of 10 nF. Neither ofthese hydrophones have built-in preamps. The preamps are incorporated inthe electronics housing 5, where they can be more easily accessed foradjustment and replacement, if necessary.

In a preferred embodiment, the three subsidiary hydrophone tubes 42, 43,44 are made from polyvinylidene difluoride (PVDF) tubing. PVDF plasticsare heavier than water but hydroacoustically transparent.

An appropriate material for constructing the fore-structure 20 andaft-structure 30 is FIBRE GLAST 2060 epoxy resin used with a mix ofcarbon composite and fiberglass cloths for the fairing sections. Manyepoxies have similar characteristics to plastics that have good acousticproperties and these materials exhibit high strength-to-weightcharacteristics.

In a preferred embodiment, the electronics housing 5 is a pressurebalanced, Castor-oil filled nylon plastic housing. Castor oil is a closeacoustic impedance match to seawater, and is an environmentally safe oilthat is readily available. The electronics housing 5 can be milled fromblack 6/6 nylon rod, and an acrylic cover can be machined from 0.5″acrylic sheet.

Four two-stage preamplifiers, one for each hydrophone, are provided inthe electronics housing 5. The two-stage preamps are all differentialinput and differential output. The first preamplifier stage has a gainof 40 dB, and the second stage has a gain of 21 dB, for a total gain of61 dB. The 3 dB bandwidth of the preamps is 3.0 kHz to 200 kHz.

Also provided are two linear regulators that accept the +/−12 VDC fromthe topside batteries and regulates down to the +/−3.0 VDC used in thelow noise preamps. 12 VDC power is provided to the pressure-depth sensor6 and the orientation sensor 7.

A pressure-depth sensor 6 is provided to measure, in real time, thewater pressure, from which the depth can be derived. The depth must beknown in order to locate the ship-towed volumetric array inthree-dimensional space using topside GPS data and knowledge of thelength of the tow cable and the direction of movement. Sound travels inwater through pressure waves, which the hydrophones can detect. Thepressure-depth sensor 6 also provides real-time measurements of thebaseline or background pressure in which the hydrophones are operating.An appropriate pressure sensor is the HONEYWELL PX2AS1XX250PACHX, whichhas a 4-20 mA loop output and a full scale range of 250 psi. It ispossible that the ship-towed volumetric hydrophone array 10 could reachthis depth and pressure if the vessel ever stops in deep water with thearray deployed with a 300 meter cable. This sensor will not be destroyedshould this happen.

An orientation sensor 7 measures, in real time, any roll, yaw, or pitchin the ship-towed hydrophone volumetric array's attitude along threeaxes of acceleration. Precise knowledge of the orientation is necessaryin order to know the positioning of the hydrophones each with the othersso that meaningful directional locations of sound sources can becalculated. The ship-towed hydrophone volumetric array is designed to bestable and stay in trim while being towed, but some variation inorientation will likely occur and must be measured in real time andfactored into the analysis of hydrophone data. The orientation sensor 7is ideally placed at the projected center point of the tetrahedronformed by the four hydrophones. An appropriate orientation sensor is anIAC-I-03 that provides three axes of acceleration with 4-20 mA loopsignaling.

Referring to FIG. 12, an exploded view further illustrates the internaland external components of the ship-towed volumetric hydrophone array 10system and their spatial relationships each to the other.

Referring to FIG. 13, with the ship-towed hydrophone volumetric array 10traveling in trim, with its central axis or roll axis in line with thedirection of travel, the orientation sensor 7 reports zero degreesdeviance about all three axes.

Referring to FIG. 14, with the ship-towed hydrophone volumetric array 10rolling, the orientation sensor 7 reports the roll, and theconsequential shifting of the hydrophones 1, 2, 3, 4 is available to beaccounted for in the analysis of data.

Referring to FIG. 15, with the ship-towed hydrophone volumetric array 10yawing, the orientation sensor 7 reports the yaw, and the consequentialshifting of the hydrophones 1, 2, 3, 4 is available to be accounted forin the analysis of data.

Referring to FIG. 16, with the ship-towed hydrophone volumetric array 10pitching, the orientation sensor 7 reports the pitch, and theconsequential shifting of the hydrophones 1, 2, 3, 4 is available to beaccounted for in the analysis of data.

Referring to FIG. 17, a schematic view of the topside electronics 90aboard the towing ship, the system is receiving signals from a GPSantenna 91 and communicating with the hydrophone volumetric arraythrough a multi-channel power and data cable 92. The signals beingreceived from the hydrophone array and the calculated, derivedthree-dimensional location of the sound source are displayed in realtime and are recorded for further analysis and comparisons.

Many changes and modifications can be made in the present inventionwithout departing from the spirit thereof. I therefore pray that rightsto the present invention be limited only by the scope of the appendedclaims.

We claim:
 1. A low-drag ship-towed hydrophone volumetric array for underwater deployment of an array of hydrophones towed with a tow cable having an incorporated power and data cable, the low-drag ship-towed hydrophone volumetric array comprising: (i) a streamlined fore-structure adapted to move through water with stability and without turbulence, comprising: (a) a main nose cone defining a central axis along a direction of travel; (b) a fore-and-aft fore-fin attached at a first end to said main nose cone perpendicular to the central axis, at the 180-degree position; (c) a fore-and-aft nose cone attached at a second end of said fore-and-aft fore-fin and parallel to said main nose cone; (d) a port fore-fin attached at a first end to said main nose cone perpendicular to the central axis, at the 240-degree position; (e) a port nose cone attached at a second end of said port fore-fin and parallel to said main nose cone; (f) a starboard fore-fin attached at a first end to said main nose cone perpendicular to the central axis, at the 60-degree position; and (g) a starboard nose cone attached at a second end of said starboard fore-fin and parallel to said main nose cone; (ii) a streamlined mid-structure smoothly attached to and continuing said fore-structure, comprising: (a) a main tube housing continuing said main nose cone along the central axis; (b) a fore-and-aft hydrophone tube of hydroacoustically transparent material continuing said fore-and-aft nose cone; (c) a port hydrophone tube of hydroacoustically transparent material continuing said port nose cone; and (d) a starboard hydrophone tube of hydroacoustically transparent material continuing said starboard nose cone; (iii) a streamlined aft-structure smoothly attached to and continuing said mid-structure, comprising: (a) a main open aft collar continuing said main tube housing along the central axis; (b) a fore-and-aft aft fin attached at a first end to said main open aft collar perpendicular to the central axis, at the 180-degree position; (c) a fore-and-aft open aft collar attached at a second end of said fore-and-aft aft fin and parallel to said main open aft collar; (d) a port aft fin attached at a first end to said main open aft collar perpendicular to the central axis, at the 240-degree position; (e) a port open aft collar attached at a second end of said port aft fin and parallel to said main open aft collar; (f) a starboard aft fin attached at a first end to said main open aft collar perpendicular to the central axis, at the 60-degree position; and (g) a starboard open aft collar attached at a second end of said starboard aft fin and parallel to said main open aft collar; (iv) a fore hydrophone mounted in the forward portion of said fore-and-aft hydrophone tube and defining one point of a tetrahedron centered along the central axis; (v) an aft hydrophone mounted in the rearward portion of said fore-and-aft hydrophone tube at another point of the tetrahedron; (vi) a port hydrophone mounted in the center portion of said port hydrophone tube at another point of the tetrahedron; (vii) a starboard hydrophone mounted in the center portion of said starboard hydrophone tube at another point of the tetrahedron; where said hydrophones are held in fixed, equidistant, tetrahedral positions one to another and to the central axis; (viii) a pressure-depth sensor mounted in said main tube housing adapted to provide pressure data from which depth can be derived; (ix) an orientation sensor mounted in said main tube housing, along the central axis, adapted to provide roll, yaw, and pitch data from which differences in each hydrophone's spatial orientation with reference to a given sound source can be calculated; (x) an electronics housing mounted in said main tube housing, adapted to supply power to and receive data from said hydrophones, pressure-depth sensor, and orientation sensor, and transmit such data to a towing ship through a power and data cable incorporated into a tow cable; and (xi) a tapered cable clamp mounted within said main nose cone, adapted to securely fasten to a tow cable having an incorporated power and data cable; where said ship-towed hydrophone volumetric array provides data whereby any single sound in the relevant frequency range from any underwater sound source can be accurately located in three-dimensional space in real time.
 2. The ship-towed hydrophone volumetric array of claim 1, where said ship-towed hydrophone volumetric array is used in line-intersect or line-transect sampling of marine mammal populations.
 3. The ship-towed hydrophone volumetric array of claim 1, where said fore-fins and aft-fins possess the streamlined profile of the NACA 0012 standard.
 4. The ship-towed hydrophone volumetric array of claim 1, where said main nose-cone possesses a spherically blunted tangent ogive profile.
 5. The ship-towed hydrophone volumetric array of claim 1, further comprising fairing at the points of attachment of said main nose cone, said fore-fins, and said subsidiary nose cones.
 6. The ship-towed hydrophone volumetric array of claim 1, further comprising a cable-connector fairing boot covering said tapered cable connector on said main nose cone.
 7. The ship-towed hydrophone volumetric array of claim 1, further comprising fairing at the points of attachment of said main open aft collar, said aft-fins, and said subsidiary open collars.
 8. The ship-towed hydrophone volumetric array of claim 1, where said orientation sensor is a three-axis accelerometer.
 9. The ship-towed hydrophone volumetric array of claim 1, where said pressure-depth sensor is capable of functioning at high pressures in deep water.
 10. The ship-towed hydrophone volumetric array of claim 1, where said electronics housing further transmits data in a format compatible with existing NOAA cabling.
 11. The ship-towed hydrophone volumetric array of claim 1, where said electronics housing further transmits data in a format compatible with open-source PAMGUARD analysis software.
 12. The ship-towed hydrophone volumetric array of claim 1, where the tetrahedral positioning of said hydrophones places each at a 50 centimeter distance from the others.
 13. The ship-towed hydrophone volumetric array of claim 1, where the diameter of said main tube housing is 6 centimeters.
 14. The ship-towed hydrophone volumetric array of claim 1, where the diameter of said fore-and-aft hydrophone tube, port hydrophone tube, and starboard hydrophone tube are 5 centimeters.
 15. The ship-towed hydrophone volumetric array of claim 1, where said orientation sensor is placed at the center point of the tetrahedron defined by said hydrophones.
 16. The ship-towed hydrophone volumetric array of claim 1, where said hydrophone tubes are made from PVDF tubing.
 17. The ship-towed hydrophone volumetric array of claim 1, where said main tube housing is made from PVDF tubing.
 18. The ship-towed hydrophone volumetric array of claim 1, where said fore-structure is made from epoxy resin with a mix of carbon composite and fiberglass cloth.
 19. The ship-towed hydrophone volumetric array of claim 1, where said aft-fins are made from epoxy resin with a mix of carbon composite and fiberglass cloth.
 20. The ship-towed hydrophone volumetric array of claim 1, where said ship-towed hydrophone volumetric array is capable of being towed at 10 knots with a 300 meter tow cable. 